ALBERT

Description: Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~ 30 °S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of

Description: The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of  7 ‰; δ18O 〉 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of  4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of 〉 2 µmol kg−1 and a relatively low Δ(15–18) value of

Description: The University of Hamburg is part of environmental studies in the INDEX Program, which was establishes by the BGR (Federal Institute of Geosciences and Natural Resources) in Hanover to explore Massive Sulphides with regard to a potential future deep sea mining. The INDEX license area is located in the oligotrophic subtropical gyre of the South Indian Ocean. The water samples were collected with a CTD water rosette during two ship cruises with R/V Merian in 2016 (MSM 59/2 "INDEX 2016-2"; November−December 2016) and R/V Sonne in 2017 (SO 259 "INDEX 2017"; August−October 2017) and were analysed for nutrients and stable isotopes of nitrate.

Description: We investigated nutrient input and retention in the Elbe River (Germany) at the river/estuarine transition with high agricultural loads of nitrogen. Surface water samples were taken at the weir Geesthacht (stream kilometre 585, 53°25'31''N, 10°20'10''E) from 2011 to 2021. In these samples, we analyzed nutrient concentrations, nitrate dual stable isotopes and suspended particulate matter composition. Usually, samples were taken once or twice per month. Aims of the study were to investigate 1) nitrate retention in the Elbe River and catchment, 2) seasonal dynamic of nitrate stable isotopes and 3) key nitrogen turnover processes and their respective controls over a ten year period.

Description: R/V Meteor cruise M131 carried out a physical oceanography research program with a biogeochemical sampling component in the South Equatorial Atlantic Ocean and eastern boundary upwelling region off Angola and Namibia. The program was part of the EU collaborative project PREFACE (“Enhancing prediction of tropical Atlantic climate and its impacts”) and the sampling for and analysis of nutrient data were linked to BMBF collaborative project GENUS (“Geochemistry and Ecology of the Namibian Upwelling System”). CTD data of the expedition M131 are archived under doi:10.1594/PANGAEA.910994. The bottle files corresponding to water samples analysed for nutrient concentrations and nitrate isotopic composition were provided by Gerd Krahmann (GEOMAR). Depths and data for temperature, salinity, oxygen concentrations labeled “CTD“ in the table are values from calibrated CTD sensors at closure of the bottles, numbers for fluorescence are uncalibrated. Water samples were taken by Maria-Elena Vorrath during M131 and were analyzed after shipment in the laboratory of Kirstin Dähnke at Helmholtz-Zentrum Geesthacht. Nutrient concentrations were measured with an AutoAnalyzer 3 system (Seal Analytics) using standard colorimetric methods by Markus Ankele. Nitrate was determined after reduction to nitrite, followed by a reaction with sulfanilamide to form a red azo dye (Grasshoff and Anderson 1999). Phosphate was measured after formation of a blue antimony-phosphorous colour complex, according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). The relative errors of duplicate sample measurements were below 1.5% for NOx and phosphate concentrations, and below 0.3% for ammonium and silicate. The detection limit was 〈0.5 µmol kg−1 for NOx, 〉 0.1 µmol kg−1 for phosphate, 〉0.013 µmol kg−1 for ammonium and 〉0.016 µmol kg−1 for silicate. Delta15N_NO3 and Delta18O_NO3 were determined in the laboratory at Helmholtz-Zentrum Geesthacht (Kirstin Dähnke) with the denitrifier method (Sigman et al. 2001; Casciotti et al. 2002). The isotopic composition was determined with a GasBench II coupled to a Delta Plus XP mass spectrometer (ThermoFinnigan). Replicate measurements were performed, and two international standards (IAEA-N3, Delta15N = 4.7‰, Delta18O=25.6‰ and USGS 34 (Delta15N=-1.8‰, Delta18O=-27.9‰; Böhlke et al. 2003), were measured with each batch of samples. To correct for exchange with oxygen atoms from water, a bracketing correction was applied (Sigman et al. 2009). The standard deviation for standards and samples was 0.2‰ for Delta15N and 0.4‰ for Delta18O.

Description: The University of Hamburg is part of environmental studies in the INDEX Program, which was establishes by the BGR (Federal Institute of Geosciences and Natural Resources) in Hanover to explore Massive Sulphides with regard to a potential future deep sea mining. The INDEX license area is located in the oligotrophic subtropical gyre of the South Indian Ocean. The water samples were collected with a CTD water rosette during two ship cruises with R/V Merian in 2016 (MSM 59/2 "INDEX 2016-2"; November−December 2016) and R/V Sonne in 2017 (SO 259 "INDEX 2017"; August−October 2017) and were analysed for nutrients and stable isotopes of nitrate.